In the field of quantum cryptography, there is a need to reliably produce single photons. One photon source which has been previously suggested for producing single photons is based on quantum dots. In a quantum dot, an exciton is formed when there is a bound state between a small number of electrons in the conduction band and holes in the valence band, radiative decay occurring when one hole and one electron recombine resulting in the emission of a photon. Due to the generation process, the time at which the photon is emitted can be carefully controlled and such a process cannot give rise to the emission of two photons at the same time due to the Pauli Exclusion principle. Each combination of electrons and holes has different energy differences between the initial and final electronic states, leading to photons of different wavelengths, which are by convention labelled with the name of the initial state. As an example, if the dot contains two electrons and one hole, it can radiatively decay to emit a single photon labelled as the “negatively charged exciton”, leaving one electron remaining in the dot. Such a transition is preferred for generation of coherent single photons as this transition is empirically observed to be most robust to decoherence.
In the field of quantum cryptography, quantum imaging and quantum computing there is also a need to produce pairs of photons. Such photons can be created from a cascade emission process in single quantum dots initially filled with two electrons and two holes, a “biexciton state”. This state can emit “a biexciton photon” leaving one electron and one hole in a “(charge-neutral) exciton” state. This electron and hole then recombine to emit an “exciton” photon leaving the dot empty. Through control of the properties of the exciton state these two photons can be entangled.
A particularly popular material system is the formation of InAs quantum dots in GaAs. The emission wavelength may be tuned by applying an electric field across the quantum dot. However applying an electric field across the quantum dot causes the efficiency of the photon source to decrease since the applied field enhances tunneling of carriers out of the quantum dot before photon emission can occur.